This invention relates to compensation of the bias current to a giant magnetoresistive (GMR) head, and particularly to a method and apparatus for measuring temperature within a magnetic disc drive and compensating the bias current based on the temperature.
Magnetic disc drives employ giant magnetoresistive (GMR) heads to detect data recorded on magnetic discs confronting the heads. More particularly, variations in the magnetic field representing data alter the resistance of the GMR head field as the disc moves past the head. Application of a bias current to the GMR element generates a voltage across the head that varies in accordance with the changing magnetic field. This changing voltage is presented to a read recovery circuit. For a given bias current, the voltage value is based on the resistance of the GMR element. Hence, fluctuations in the voltage value, representing data, are dependent upon fluctuations of the GMR resistance value. While the read voltage generated by the GMR head is independent of the velocity of the disc medium, it is dependent upon the quality of data written on the disc and the fly height of the GMR head, both of which are dependent on the temperature of the disc drive.
Disc drive manufacturers employ a fixed bias current to the GMR head to produce the sense voltage to the read and servo circuits. The bias current is selected during manufacture and is fixed over the entire operating range of the disc drive. At cold temperatures, below about 18xc2x0 C., the GMR head tends to fly higher (more distant) from the disc, thereby detecting less magnetic flux due to data on the disc. As a result, the voltage due to data produced by the head is smaller at lower temperatures than at higher temperatures (above about 46xc2x0 C.). Moreover, the write head of the disc drive does not overwrite prior data or write transition parameters as well as at lower temperatures as at higher temperatures. Consequently, the quality of write transition parameters and overwrite is not as good when written at low temperatures as when written at warmer temperatures.
At higher operating temperatures of the disc drive (above about 46xc2x0 C.), there is a risk of overheating the GMR element. This is because power is not as easily dissipated at higher temperatures as at lower temperatures, leading to overheating of the head. Moreover, at higher temperatures, the head tends to xe2x80x9cflyxe2x80x9d lower (closer to the disc), thereby detecting more magnetic flux, tending to increase head resistance and leading to greater heating of the GMR element. Thus, there is a risk of overheating the GMR element at higher temperatures, resulting in a lower life span for the head.
The present invention is directed to a technique for run time temperature compensation of the bias current for a magnetoresistive head. Particularly, the present invention operates the head with a higher bias current at cold temperatures and a lower bias current at warmer temperatures.
In one embodiment of the invention, a method of run-time temperature compensating bias current to a magnetoresistive read head of a disc drive is provided. Temperature within the drive is sensed, and periodically an analog-to-digital converter of the recovery circuit derives a digital representation of the temperature within the disc drive. A bias current value is identified, based on the representation of temperature. The bias current source is set to the identified bias current value.
In one form of the invention, the bias current value is identified by defining a plurality of temperature ranges for temperature within the disc drive, and a bias current value for each temperature range. A hysteresis range of temperatures is defined for each boundary between temperature ranges. A repetitive process measures temperature within the disc drive and identifies the temperature range in which the drive is operating. The bias current is set based on either the identified temperature range if the measured temperature is not in a hysteresis range, or the bias current value previously set if the measured temperature is in a hysteresis range.
In another embodiment of the invention, a disc drive has a housing containing a disc for storing data, a magnetoresistive read head responsive to stored data to supply analog read signals representing the stored data, a temperature sensor for supplying an analog temperature signal representing temperature in the housing; and a read recovery circuit connected to the head. The read recovery circuit includes an analog-to-digital converter for converting the analog read signals to digital read signals. An interrupt processor periodically couples the temperature sensor to the analog-to-digital converter to derive a digital representation of the temperature within the disc drive. A table contains digital representations of bias current values corresponding to temperature ranges. A bias current value is selected from the table corresponding to the temperature within the disc drive. A source of bias current is responsive to the selector to supply bias current to the head having a value selected by the selector.
In one form of the apparatus, the interrupt processor operates the analog-to-digital converter to update temperature measurements when the head confronts a selected servo wedge and upon a predetermined number of revolutions of the disc. In one form, the interrupt processor initiates temperature measurement update once each 256 revolutions of the disc (about 2.84 seconds for a 5,400 rpm disc).